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<div class="titlepage"><div><div><h3 class="title">
<a name="phoenix.starter_kit.values"></a><a class="link" href="values.html" title="Values">Values</a>
</h3></div></div></div>
<p>
        Values are functions! Examples:
      </p>
<pre class="programlisting"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span>
<span class="identifier">val</span><span class="special">(</span><span class="string">"Hello, World"</span><span class="special">)</span>
</pre>
<p>
        The first evaluates to a nullary function (a function taking no arguments)
        that returns an <code class="computeroutput"><span class="keyword">int</span></code>, <code class="computeroutput"><span class="number">3</span></code>. The second evaluates to a nullary function
        that returns a <code class="computeroutput"><span class="keyword">char</span> <span class="keyword">const</span><span class="special">(&amp;)[</span><span class="number">13</span><span class="special">]</span></code>, <code class="computeroutput"><span class="string">"Hello,
        World"</span></code>.
      </p>
<h5>
<a name="phoenix.starter_kit.values.h0"></a>
        <span class="phrase"><a name="phoenix.starter_kit.values.lazy_evaluation"></a></span><a class="link" href="values.html#phoenix.starter_kit.values.lazy_evaluation">Lazy
        Evaluation</a>
      </h5>
<p>
        Confused? <code class="computeroutput"><span class="identifier">val</span></code> is a unary
        function and <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> invokes
        it, you say? Yes. However, read carefully: <span class="emphasis"><em>"evaluates to a
        nullary function"</em></span>. <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code>
        evaluates to (returns) a nullary function. Aha! <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code>
        returns a function! So, since <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code>
        returns a function, you can invoke it. Example:
      </p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
        (See <a href="../../../../example/values.cpp" target="_top">values.cpp</a>)
      </p>
<div class="blurb">
<div class="titlepage"><div><div><p class="title"><b></b></p></div></div></div>
<p>
        <span class="inlinemediaobject"><img src="../../images/tip.png"></span>
        Learn more about values <a class="link" href="../modules/core/values.html" title="Values">here.</a>
      </p>
</div>
<p>
        The second function call (the one with no arguments) calls the nullary function
        which then returns <code class="computeroutput"><span class="number">3</span></code>. The need
        for a second function call is the reason why the function is said to be
        <span class="bold"><strong><span class="emphasis"><em>Lazily Evaluated</em></span></strong></span>. The
        first call doesn't do anything. You need a second call to finally evaluate
        the thing. The first call lazily evaluates the function; i.e. doesn't do
        anything and defers the evaluation for later.
      </p>
<h5>
<a name="phoenix.starter_kit.values.h1"></a>
        <span class="phrase"><a name="phoenix.starter_kit.values.callbacks"></a></span><a class="link" href="values.html#phoenix.starter_kit.values.callbacks">Callbacks</a>
      </h5>
<p>
        It may not be immediately apparent how lazy evaluation can be useful by just
        looking at the example above. Putting the first and second function call
        in a single line is really not very useful. However, thinking of <code class="computeroutput"><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">)</span></code> as a callback function (and in most cases
        they are actually used that way), will make it clear. Example:
      </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">F</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">print</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span>
<span class="special">{</span>
    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">f</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">int</span>
<span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">print</span><span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="number">3</span><span class="special">));</span>
    <span class="identifier">print</span><span class="special">(</span><span class="identifier">val</span><span class="special">(</span><span class="string">"Hello World"</span><span class="special">));</span>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
        (See <a href="../../../../example/callback.cpp" target="_top">callback.cpp</a>)
      </p>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2010, 2014, 2015 Joel de Guzman, Dan Marsden, Thomas
      Heller, John Fletcher<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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